Many patents have issued on breast specific coils for magnetic resonance imaging (MRI). See, for example, U.S. Pat. Nos. 7,084,631 and 7,970,452. Some of these breast specific coils are designed to improve imaging by increasing signal-to-noise ratio (SNR) by adding coils or by facilitating repositioning coils. Some of these breast specific coils are suitable for MRI-guided surgical procedures.
Conventional breast specific coils may include plates that can be moved to immobilize a breast and some even include coils that can be added or removed. Some conventional breast specific coils include center supports that may include RF coils for bilateral imaging. While these conventional breast specific coils have improved breast imaging and have improved MRI-guided procedures, further improvements are still sought.
MRI detects the nuclear magnetic resonance (NMR) signals produced by protons in the presence of a strong magnetic field after specific excitation by radio frequency (RF) energy. The NMR signals are detected by antennae known as “coils.” In different usages, the term “coil” may refer to just the antenna, or may refer to the antenna, its housing, and support structure. The term “coil” may refer to an assembly that includes two or more coils. An operable part of the coil may be referred to as a “coil element.” The operable part may also be referred to as the coil.
MRI involves sampling in k-space to acquire an NMR signal from an object exposed to magnetic fields, gradients, and RF energy produced by an MRI apparatus. The quality of a magnetic resonance image may depend, at least in part, on the proximity of the apparatus (e.g., coils) producing the fields, gradients, and RF energy to which the object being imaged is subjected. The quality may also depend on the number, proximity, and orientation of coils receiving NMR signals from an object. MRI is frequently used for diagnostic medical imaging. Recently, MRI has also been used to guide surgical techniques. For example, MRI has been used to guide needle biopsies.
The quality of the NMR signal received from an object being imaged may be described, at least in part, by its SNR. One goal in an MRI session is to have a good (e.g., high) SNR. SNR is a function of several factors. One of the factors includes how close a coil is located to the object being imaged. Theoretically, a separate individual coil could be fashioned for each MRI session to account for differences between patients. Practically, this is unlikely due to both time and cost constraints. Therefore, one-size-fits-all coils are generally employed, or a very small set of different sized coils (e.g., adult, child) may be employed. Unfortunately, one-size-fits-all coils generally yield poor (e.g., low) SNR. Additionally, the need to accommodate access to a breast for an interventional device, such as a biopsy needle, during breast MRI may also produce a competition between proximity for high SNR and spacing to allow access.
Breast MRI has become increasingly important over time. Thus, numerous patents have been issued in this space. For example, U.S. Pat. No. 7,084,631 describes an MRI array coil system for breast imaging. The coil system includes top and bottom openings for receiving and supporting breasts, and side windows for accessing the breasts from the side while the patient is positioned on the apparatus.
U.S. Pat. No. 7,970,452 describes an open architecture imaging apparatus and coil system for MRI. The '452 patent describes an apparatus where RF coils and compression plates can be positioned, repositioned, held in place, and otherwise manipulated to provide improved SNR and an improved patient experience. The '452 patent describes a separable and reconfigurable coil system that may be optimized for particular imaging purposes including, but not limited to, bilateral imaging, unilateral imaging, imaging of the chest wall for mastectomy patients, and interventional procedures. The '452 patent recites that “a fundamental aspect to this disclosure of technology is the separation of patient support structures from RF coil system.” (Column 7, lines 18-20). The '452 patent also recites how “the ability to accept modular coil elements in an interchangeable support structure is a unique aspect of the present invention.” (Column 10, lines 64-66). Thus, some patents have described modular coil elements that can be added or removed from a larger patient support structure.
Some conventional apparatus exist that employ the one-size-fits-all approach for supporting needle biopsies for acquiring breast tissue. The one-size-fits-all approach to coil design for breast imaging may lead to sub-optimal results. Conventional apparatus may include a one-size-fits-all coil and a one-size-fits-all biopsy plate.
FIG. 1a illustrates a front view of a coil 100. FIG. 1b illustrates a side view of coil 100. Coil 100 includes a housing 110 and an operational part 120. The operational part 120 may include copper wire or traces and attached circuitry. The wire and the circuitry may operate as a transceiver that can both transmit RF energy into an object to be imaged and receive NMR signals resulting from the application of that RF energy. How close the copper wire can be placed to an object to be imaged is a function of at least the thickness T1 of the housing 110. Conventionally, coil 100 has been a monolithic item that can be positioned or manipulated as a single item.
FIG. 2a illustrates a front view of a biopsy plate 200. FIG. 2b illustrates a side view of biopsy plate 200. FIG. 2c illustrates a front view of another biopsy plate 220. Biopsy plate 200 is illustrated being divided into sixteen regions. These regions may be fashioned by internal dividers. Biopsy plate 200 may be made, for example, from plastic. During a surgical procedure (e.g., biopsy), biopsy plate 200 is likely going to come in contact with biological fluids, including blood. Therefore a separate biopsy plate 200 will likely be employed for each biopsy and the biopsy plate 200 will likely be cleaned and sterilized or discarded after use.
The sixteen regions may measure, for example, one inch by one inch, two centimeters by two centimeters, or other dimensions. A needle biopsy may need greater precision for positioning a needle than can be provided by a one inch by one inch opening. Therefore, a needle positioning block 210 may be positioned in one of the sixteen regions. The needle positioning block 210 is also illustrated with sixteen regions. While sixteen regions are illustrated in both biopsy plate 200 and needle positioning block 210, different numbers of regions may be found in different biopsy plates and in different needle positioning apparatus. In second biopsy plate 220, one of the regions houses the needle positioning block 210. In a needle biopsy, the needle would be inserted to a desired depth into a volume (e.g., breast) after a location within the volume was identified during imaging. The position and direction of travel of the needle is controlled by the region in the needle positioning block 210 through which the needle is inserted. Both the biopsy plate 200 and the needle positioning block 210 are likely to be cleaned and sterilized or discarded after a procedure.
FIG. 3 illustrates a side view of a coil 300 paired with a biopsy plate 310. Coil 300 includes an operational part 301 and a housing 302. The biopsy plate 310 is illustrated with a needle positioning block 311 housed in one of the regions in biopsy plate 310. During the image acquisition portion of an MRI-guided needle biopsy, the coil 300 and the biopsy plate 310 may be positioned close together and as close to a breast as possible. How close the coil 300 can be placed to the volume to be imaged depends on the thickness T3 of the housing of the coil 300 and the thickness T4 of the biopsy plate 310. In one embodiment, when the imaging portion of the MRI-guided needle biopsy is complete, the coil 300 may be removed to provide access to the biopsy plate 310 and then the needle positioning block 311 may be inserted into the biopsy plate 310 at a relevant location. In one embodiment, during a first imaging portion of a biopsy procedure, the side coil 300 may be in position and the biopsy plate 310 may not be in position. For example, the side coil 300 may be mounted in the apparatus while the biopsy plate 310 is not mounted in the apparatus. After the first imaging portion, the side coil 300 may be removed from the apparatus and replaced with the biopsy plate 310.
A biopsy plate is likely to come in contact with biological fluid during a biopsy. Therefore, it is likely that the biopsy plate will need to be destroyed or at least rigorously cleaned or sanitized after use. Coils are expensive. Coils are also generally housed in a solid housing. Therefore, it is unlikely that a needle will be pushed through a coil, both because it would damage the coil and because it would be undesirable to have the coil come in contact with the biological fluid. Therefore, during an MRI-guided biopsy, the coil may first be positioned beside the biopsy plate to facilitate acquiring an image and registering the image to the biopsy plate. After the image is acquired, the coil may be removed so that the needle can be pushed through the biopsy plate. Positioning the coil and then removing the coil requires skilled operator attention, and thus takes time. Also, as described above, the distance between the coil and the volume to be imaged affects the SNR for signal acquired from the object being imaged. In general, having the coil closer to the volume during an MRI procedure improves SNR while having the coil farther from the volume negatively impacts SNR.
SNR may also depend on the number of coils and the orientation of the coils used to image a volume. The anatomy of a volume to be imaged may control both the number of coils that can be used and the proximity of those coils. For example, it is possible to surround a knee with a number of coils and to bring those coils into very close proximity with the knee. However, for an image guided needle biopsy of a breast, it may not be possible to surround the volume and it may not be possible to bring the coils as close to the volume as desired due to the requirement of fixing the volume with the biopsy plate.
FIG. 4a illustrates a view looking towards the feet from the head of a patient of portions of an apparatus 405. FIG. 4b illustrates a top view of apparatus 405. Apparatus 405 may support a patient who is lying face down during an MRI-guided needle biopsy. Apparatus 405 includes housing 410, a biopsy plate 420, and a central fixed element 430. Apparatus 405 includes an opening 499 through which breast 400 may hang. Apparatus 405 may have two such openings. In one embodiment, the central fixed element 430 may not be present.
FIG. 4a also represents a breast 400 as it might appear when viewed from the head of a patient that is lying face down on apparatus 405. Initially, the breast 400 would hang down through opening 499 in the apparatus 405. In the initial positioning, breast 400 might be able to move, making it difficult, if even possible at all, to accurately place a needle into a region of interest identified during an MRI-guided biopsy. Therefore, before imaging, the breast 400 may be compressed into a different shape by being squeezed between biopsy plate 420 and another fixed element 430. In one embodiment, the biopsy plate 420 may be replaced with a side coil and the breast may be squeezed between a side coil and another fixed element 430. After imaging, the side coil may be replaced with the biopsy plate 420. In one embodiment, the biopsy plate 420 may squeeze the breast in the same directions and with the same forces as the side coil did.
FIG. 5a illustrates a view looking towards the feet from the head of a patient of portions of an apparatus 505. FIG. 5b provides a top view of apparatus 505. Apparatus 505 may support a patient who is lying face down using support structure 510. Apparatus 505 includes a biopsy plate 520 and a central fixed element 530. Apparatus 505 includes an opening 599 through which breast 500 may hang. Apparatus 505 may have two such openings.
FIG. 5a also represents breast 500 as it might appear when viewed from the head of a patient that is lying face down on apparatus 505. The breast 500 is illustrated after it has been compressed between biopsy plate 520 and fixed element 530. A coil 540 is illustrated beside biopsy plate 520. The breast 500 may be imaged using coil 540. Other coils may also be involved in imaging breast 500. Note that as biopsy plate 520 compresses breast 500, breast 500 is moved farther from coil 540. With the breast 500 compressed into a shape that can be maintained during imaging and then during needle insertion, the imaging may proceed. In one embodiment, a two-step process may be performed. First, the breast may be compressed and imaged. Second, the side coil may be removed to provide access to the biopsy plate.
FIG. 6 illustrates breast 500 as it might appear when viewed from the head of a patient that is lying face down on apparatus 505. The breast 500 is illustrated after a region of interest 550 has been identified. A needle 560 may be inserted through the biopsy plate 520 to acquire tissue from the region of interest 550. A needle positioning block may be positioned in the biopsy plate 520 before the needle 560 is inserted to facilitate more accurate placement of the needle 560. The coil 540 may be removed before the needle 560 is inserted into the region of interest 550.